The present invention generally relates to a method and apparatus for structural repair and, more particularly, to a method and apparatus for repairing a damaged or defective area of a structure.
It is well known to implement patch repairs to damaged composite structures (such as aircrafts and/or spacecrafts) that are generally made up of sandwich reinforced materials having outer fiber-reinforced composite layers separated by a lightweight core made up of metallic or non-metallic honeycomb, structural foams, and/or wood fibers (e.g. balsa). Generally, damage areas (usually recesses) are formed in these composite structures as a result of fabrication defect(s) or impact(s) from an object. These recesses usually extend at least through an outer composite layer and into the central core of the composite structure.
Conventional repair of these composite structures generally involves drilling holes through the damaged outer composite layer and subsequently injecting adhesive and/or filler material into the core through these holes. This injection of adhesive is generally followed by bonding a pre-cured or co-cured patch to the repair area. While the adhesive which holds the patch to the repair area is drying, the patch is usually held in position using one or both a vacuum bag and tape. Use of the vacuum entails sealing the vacuum bag around the repair area and changing the pressure to approximately 1 atmosphere (up to 14.7 psi); while use of the tape to hold the patch in place should be obvious to those of ordinary skill in the art.
The use of tape and vacuums to provide compressional force on the patch has demonstrated ineffective results when these repair areas were tested for sheer and peel strength. Such ineffectiveness is due, at least in part, to conventional tape and vacuum methods only providing up to about 14.7 psi of force at sea level (and less at higher altitudes). Maximum bond strength cannot be attained at these conventional levels of compressional force. Further, it has been shown that the use of vacuum bags can reduce the bond strength of the adhesive between the patch and the repair area by up to 35%. Such a reduction in bond strength is due to incomplete chemical reactions undergone by the adhesives in a vacuum environment.
Other methods have attempted to increase compressional forces, but have sacrificed structural integrity of the original composite structure in the process. For example, one method consists of drilling a hole through the entirety of the composite structure and creating a vice-type apparatus to increase the compressional force on the patch. Such a method obviously results in an undesirable hole through the entirety of the composite structure. In practical terms, drilling a hole from the outside of an airplane into the fuselage would not promote the structural wellbeing of that aircraft. Yet another method of repair utilizes a plurality of pins which go through the entirety of the patch and into the composite structure. This pin method also has a drawback in that an increased number of holes are put into the outer composite layer of the structure, weakening the structure, with every attempt to strengthen the composite structure. Accordingly, the integrity of the composite structure is sacrificed every time another pin is driven into the structure. It would therefore be desirable to have a method of repairing composite structures that provides greater compressional forces than conventional methods without sacrificing additional integrity of the composite structure.
The present invention is generally directed to a method and apparatus for structural repair. More specifically, the present invention is generally directed to a method and apparatus for repairing a damaged or defective area of a structure, such as any general composite structure. The repair method and structure of the present invention desirably addresses the lack of appropriate clamping force imposed on repaired areas of composite structures using conventional repair methods. Accordingly, any appropriate type/configuration of damaged or defective structure may benefit from performing repairs on such a structure utilizing the repair method and/or apparatus of the present invention. One particularly desirable application is the repair of the outer shell of a launch vehicle, an aircraft, a spacecraft, a rocket, or any other aerodynamic body that flies.
A first aspect of the present invention includes a method for repairing a structure having a damaged or defective area. The repair method generally includes forming a recess on an exterior surface of the structure. Once the recess exists, a threaded insert is placed in the recess and a first material is deposited in the recess. The particular order in which the threaded insert and the first material are placed in the recess is not critical. This first material is generally cured after it has been placed in the recess. After both the first material and threaded insert have been positioned in the recess, a patch is usually positioned over the recess. A compression fixture is then generally placed over the patch which covers the first material-filled recess. The compression fixture is then compressed against the exterior surface of one or both the structure and the first material by directing a threaded fastener through the compression fixture and into the threaded insert.
Various refinements exist of the features noted in relation to the subject first aspect of the present invention as well. Further features may also be incorporated in the subject first aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. For instance, the recess can be formed by the outer surface of the structure obtaining a nick, notch, depression, dent, slit, cut, or the like. Some recesses can be intentionally formed and/or shaped to promote optimal repair of a damaged area of the structure. Generally, the size and shape of the original recess preferably encompasses the damaged area, but exceptions exist wherein the outer surface of the structure is augmented to change the size or shape of the original recess/damaged area. These recesses can be formed/shaped by machining, milling, hole-sawing, routing, and any other methods known in the art for forming and/or shaping a recess in a structure. Regardless of how the recess is formed, the depth of the recess is ideally limited to less than a wall thickness of the structure in which the recess is formed (i.e., the recess preferably does not extend through the wall thickness of the structure). In variations where the structure is a composite having first and second peripheral face sheets and an internal core, a residual thickness of the internal core material can be left in the recess to form a bottom surface of the recess.
Upon the threaded insert being placed in the recess in the first aspect, the threaded insert may be positioned in spaced relation to a bottom of the recess. In other words, the threaded insert may be positioned to avoid contact with the base/floor/bed of the recess. Additionally, the threaded insert may be positioned interiorly of a plane of the exterior surface of the structure. Put another way, the threaded insert may be positioned in the recess so that no portion of the threaded insert extends beyond the plane that contains the outer surface of the structure. In another embodiment, the threaded insert is positioned in the recess so that an end portion of the threaded insert and the outer surface of the structure are at least substantially coplanar. Other variations which reflect at least a portion of the threaded insert protruding from the recess and extending beyond the plane of the outer surface of the structure are contemplated.
The order in which the first material and the threaded insert are disposed in the recess in the case of the first aspect may be generally dependent upon, amongst other factors, the chemical and physical properties of the first material used. The method of the first aspect can include overfilling the recess with an amount of the first material. In other words, the amount of first material that is positioned in the recess can be greater than the volumetric space of the recess. The first material is preferably deposited around and in contact with the threaded insert. In other words, the first material touches and communicates with outer surfaces of the threaded insert to hold the insert in the recess. Preferably, the repairer tends to avoid depositing the first material in an aperture of the threaded insert during the placement of the first material and the threaded insert in the recess.
The first material used by the first aspect may be an adhesive material, and in any case is a material that is capable of holding the threaded insert in position within the recess. In one embodiment, the first material is a potting compound or any appropriate substitute which has adhesive properties as well as the ability to cure/harden. For example, the first material can include one or more epoxy adhesives. Preferably, the first material, in a cured condition, can withstand temperatures of at least about 200xc2x0 F.; and more preferably at least about 300xc2x0 F. In addition, the first material can include substantially the same or a different material than that which makes up the body of the structure. Some preferred variations of the first material are a mixture of at least one potting compound and a filler. In such preferred variations, the filler can be one or more of microballoons, microspheres, chopped fiber, other appropriate low-density material, and any combination thereof. The composition of the first material is only limited by the ability to harden/cure and the ability to hold and retain the position of the threaded insert in the recess at least during use of the compression fixture. Accordingly, the threaded insert is generally formed from a material which is receptive to adhesive properties of the first material.
After both the threaded insert and the first material are properly positioned in the recess, the first material is generally cured. xe2x80x9cCuringxe2x80x9d generally refers to one or more of a hardening, setting, and drying of the first material. The first material may be cured by exposing it to room temperature, by heating the first material, or by any other appropriate method. To promote maintaining the position of the first material in the recess, a caul sheet may be placed over the exterior surface of the first material. A caul sheet is generally defined as a rigid/semi-rigid implement which is placed over the recess (which is filled with the first material) to keep the first material in place (i.e. keep the first material from coming out of the recess). Use of a caul sheet in the repair process generally tends to provide a smooth surface to the cured first material. Generally, a vacuum bag, tape, or other appropriate temporary attaching means may then be utilized to maintain the position of the caul sheet on the first material that occupies the recess of the damaged area. Put another way, once the caul sheet is placed atop the first material-filled recess, the caul sheet may be either taped to the exterior surface of the structure, or a vacuum bag is secured to the exterior surface of the structure so that it at least encompasses the entirety of the caul sheet and the damaged area. Use of a vacuum bag to secure implements to a structure will be obvious to those of ordinary skill in the art.
After the curing process has been completed in the case of the first aspect, a peripheral surface of the cured first material which is preferably substantially aligned with the exterior surface of the structure, may contain some depressions and uneven areas. These depressions/uneven areas in the cured first material may be filled in with additional first material. Further, after the curing process, the peripheral surface of the cured first material may be smoothed (e.g., sanded down with sandpaper), for instance generally to a continuity of the exterior surface of the structure.
Once the first material in the recess is cured in the first aspect, the caul sheet and tape/vacuum bag may be removed from the repair area. The patch may be adhesively adhered to at least one of the exterior surface of the structure and the exposed peripheral surface of the cured first material so that the patch entirely encompasses at least the recess. This patch may generally be a composite material (preferably pre-cured) which is made up of one or both of carbon fiber reinforced plastic and glass fiber reinforced plastic. This patch can also be made up of a carbon/glass fiber reinforced plastic, an aromatic polyamide fiber (such as Kevlar(copyright) made by DuPont) reinforced plastic, aluminum alloy, titanium alloy, corrosion-resistant steel, or any appropriate patch material. Preferably, adhesive is spread over the substantial entirety of this patch and substantially all of the peripheral surface of the cured first material (as well as at least portions of the exterior surface of the structure which surrounds the recess), so that the adhesive-covered patch can be engaged to the adhesive-covered repair area. The side of the patch having adhesive applied to it is then at least generally positioned upon and pressed against the damaged area. However, other methods of affixing the patch at least to one or both of the peripheral surface of the cured first material and the exterior surface of the structure are contemplated, such as applying adhesive to only one of the components and/or applying adhesive to less than the entirety of the respective components, amongst others.
Generally, the patch used by the first aspect may contain a hole which is at least generally compatible in size and shape with the threaded fastener. This hole is generally lined up with the aperture of the threaded insert (which is embedded in the cured first material). The compression fixture may then be placed over the patched area. The compression fixture may be a rigid apparatus which also contains a hole compatible in size and shape with the threaded fastener. The hole of the compression fixture is aligned with the hole of the patch and the aperture of the threaded insert which is embedded in the cured first material. The threaded fastener is then directed through the hole in the compression fixture and the hole in the patch, and engaged into the aperture of the threaded insert. This threaded fastener, generally made from metal, nylon, or any other appropriate material, is then torqued/tightened into the threaded insert so that a head of the fastener draws the compression fixture toward the patched repair area. The threaded fastener is generally tightened until the compression fixture abuts and compresses the patch against at least the repair area. The amount of pressure desired from use of the compression fixture will dictate how far the threaded fastener is engaged into the threaded insert. The compression fixture is preferably compressed against one or both the patch and a portion of the structure disposed beyond a perimeter of the recess at a force of at least about 15 psi (pounds per square inch), and more preferably, at least about 30 psi. The threaded fastener is generally left securely engaged with the threaded insert until the adhesive has dried. Generally, the threaded fastener and the compression fixture are removed after the adhesive has set/dried.
The threaded fastener and compression fixture may be removed from the patched area in relation to the first aspect. A variety of steps can then be taken. For instance, a second material can be applied into the aperture in the threaded fastener (which is now fixed in the cured first material which is disposed in the recess covered by the patch) generally at most up to the point at which the aperture is filled with the second material. This second material can be the same or different than the first material. Alternatively, a low-profile filler bolt can be fitted into the aperture of the threaded insert. An end opposite a threaded end of the filler bolt can extend beyond the exterior surface of the structure, but preferably, the end opposite the threaded end of the filler bolt is substantially flush with or slightly recessed from the exterior surface of the structure. As another alternative, the aperture in the threaded insert can be left open and devoid of any filler. Additional functionality comes from repairing a structure using the method of the first aspect of the present invention in that the threaded insert in the recess can later be utilized as a point of reference for any repair of the structure. For instance, a device to assist in the subsequent repair of the structure can be attached to the structure via the threaded insert affixed in the recess of the structure.
In some variations of the first aspect, a plurality of fasteners may be installed through the patch and into one or both the cured first material within the recess and any portion of the structure disposed beyond a perimeter of the recess after the compression fixture and threaded fastener have been removed. The plurality of fasteners can include, but are not limited to, Huck-bolts, Jo-bolts, Composi-Loks, pins, and screws. In other words, the fasteners are generally xe2x80x9cblindxe2x80x9d mechanical fasteners that only need to be accessed from one end to be installed.
A second aspect of the present invention relates to a repaired structure. While the structure to be repaired is generally a launch vehicle, an aircraft, or a spacecraft, the invention can be applied to a variety of structures which preferably are fabricated at least in part from composite materials. In any event, the repaired structure includes a body having an exterior surface, a recess formed in the body, a first material disposed in the recess, and a threaded insert disposed within the first material. Those various features discussed above in relation to the above-described first aspect of the present invention may be incorporated in variations of the second aspect of the present invention as well, and in the manner noted above.